Well, then not do anything. Lay in your bed, watch shitty sitcoms, eat food, just don't do anything. It will feel unusual if you've been working for a long uninterrupted time but you need time to rest.

You remind me of a runner who just ran 10mi and is asking: "Hey, I feel tired, why is that? It seems like my legs don't really feel like moving. What should I do? Drugs? Doctors? Am I addicted to something?"

Go and rest for brain's sake. In fact, I've been doing this all Sunday -- I'm laying in my bed with a laptop, looking at the cars outside, and watching crappy sitcoms. I am forcing this lazy behavior so I can recover and have fun working tomorrow.